The present invention relates to variable displacement compressors that vary the stroke of a piston accommodated in a cylinder bore by adjusting the pressure in a crank chamber.
A variable displacement compressor allows a piston to reciprocate in a cylinder bore through rotation of a drive shaft. This compresses the gas in a compression chamber and thus discharges the gas from the compression chamber. The displacement of the compressor is varied by varying the stroke of the piston. When the gas flow rate of the compressor is relatively low, the amount of the gas passing through a suction valve correspondingly decreases. This may cause self-induced oscillation of the suction valve in a free oscillation area in which the suction valve is prevented from contacting a stopper. Such oscillation of the suction valve may vary the pressure of the gas. The pressure variation of the gas then transmits to an evaporator of an external refrigerant circuit connected to the compressor, thus generating noise.
To solve this problem, Japanese Laid-Open Patent Publication No. 2000-136776 describes a compressor that has an open degree control valve that controls the communication area of a suction line. This structure suppresses the pressure variation of gas when the gas flow rate is relatively low.
However, actuation of the open degree control valve is based on a pressure difference caused by the flow of gas in the suction line. The pressure difference becomes smaller as the gas flow rate becomes lower. This may destabilize the operation of the open degree control valve, making it difficult to suppress the pressure variation of the gas.
Also, the compressor includes a supply line that connects a crank chamber to a discharge chamber and an outlet line that connects the crank chamber to a suction chamber. The compressor controls the pressure in the crank chamber by adjusting the amount of the gas passing through each of the supply and outlet lines. The displacement of the compressor is thus controlled. The open degree of the supply passage is adjusted to bring about a rapid change of the displacement. Further, a fixed orifice is provided in a bleed passage and thus reduces the short-circuit amount (the leak amount) of the compressed gas from the crank chamber to the suction chamber. Therefore, when the compressor is being started, drainage of liquid refrigerant from the crank chamber occurs only slowly due to the fixed orifice provided in the outlet line. This may lead to evaporation of an excessive amount of liquid refrigerant in the crank chamber. The pressure in the crank chamber thus rises excessively. As a result, the displacement of the compressor reaches a sufficiently high level only with a relatively long delay, hampering the starting performance of the compressor.